Let’s say you need to sign up for a mailing list that interests you, but you’re afraid spammers might get your address. We’ll call the list “exoticflowers”. Sign up with the list using the address “johndoe+exoticflowers@gmail.com”. Email to that address will still come to your “johndoe@gmail.com” address even though the “To:” will include that “+exoticflowers” in it.